A contact of a semiconductor device has been formed in the manner as explained hereunder. After a multilayer insulating film is formed, for example, on a semiconductor substrate, a contact region is etched to form a contact aperture which reaches the semiconductor substrate. Thereafter, impurity is doped to the bottom of the aperture, in this case, to the surface of semiconductor substrate, by ion implantation and then heat processing is carried out for activation. In this case, an oxide film is formed by oxidation of the semiconductor substrate at the bottom of the aperture. Since this oxide film interferes electrical formation of ohmic contact, after this oxide film is removed by the light etching from the entire surface, conductor such as semiconductor or metal is buried to the aperture. Thereafter, the conductor buried in the aperture is connected to an interconnect layer formed at the upper layer by metal or polycide, etc.
The light etching explained above is performed using the isotropic wet etching which does not easily give damage to the underlayer semiconductor substrate. However, when a multilayer insulating film is stacked on the semiconductor substrate, shape of internal wall of the contact changes depending on difference of etching rates of respective layers of the multilayer film. Here, rises a problem that overhang of insulating film is generated when the etching rate of the upper insulating film is smaller than that of the lower insulating film, shape of contact is overhanged or when the etching rate of the intermediate insulating film is smaller than that of the lower insulating film, or cavity is generated in the intermediate layer due to the side etching when, on the contrary, the etching rate of the intermediate insulating film is larger than that of the upper and lower insulating films.